The Flat River is a tributary of the Grand River in the western part of the U.S. state of Michigan. It rises as the outflow of the six lakes system in Belvidere Township, east of Edmore in Montcalm County, at 43°26?10?N 85°08?26?W? / ?43.43611°N 85.14056°W? / 43.43611; -85.14056. It flows mostly south and slightly west through Montcalm County, Ionia County, and Kent County, flowing into the Grand River in Lowell at 42°55?38?N 85°20?18?W? / ?42.92722°N 85.33833°W? / 42.92722; -85.33833. Other cities it flows through along the way include Greenville and Belding.